Since the mid -1990s, astronomers have found thousands of exoplanets in the galaxy. But these "almost 5000 exoplanets" are ridiculously little in view of the fact that there should be billions of exoplanets in our Milky Way alone. If you take a look beyond the Milky Way, it gets worse.
As far as extragalactic planets are concerned, researchers have so far identified a single anomaly in the Andromeda Galaxy. And another possible candidate in the Whirlpool galaxy. Or evidence of many free-flying planets in a nameless galaxy about four billion light-years away. So, maybe. Because perhaps, as far as we know, there are no extragalactic exoplanets yet. It all depends on who you ask.
The Andromeda Galaxy's anomaly
"Felsenfest has so far not been an Extra -Alactic exoplanet," says Philippe Jetzer, physicist from the University of Zurich. "But there are evidence."
Jetzer's team already found such an indication in 2009: an anomaly in a gravitational lensing event in the Andromeda galaxy. The Andromeda Galaxy is our cosmically adjacent spiral galaxy – the one with which the Milky Way will one day collide and merge. Until then, it is about 2.5 million light years away from us. Philippe Jetzer and his colleagues analyzed whether it would be possible to find exoplanets in this galaxy using gravitational lensing.
With the gravitational lens effect, a light source in the background – no matter what kind of light source it is, the main thing is that it is bright – emits light that is focused by a massive object in the foreground as if by a lens. On large scales you can admire magnificent Einstein rings, or the background source even appears several times. In the case of a microlens, on the other hand, the lens would make itself felt by temporarily making the source behind it appear brighter. In principle, this method can also be used to find planets in our Milky Way that either orbit their star at a great distance or that drift lonely and alone through space without a star at all. So why not extragalactic exoplanets as well?
"It doesn't work for individual exoplanets," says Philippe Jetzer. Because the 2.5 million light years removal of Andromedagalaxy still make it impossible to really dissolve the light of individual stars. Instead, researchers would have to examine individual pixels of a recording on brightness fluctuations. A single pixel can then contain the light of several stars. After all, now, now and his colleagues came to the conclusion: the method should work. Statistics on exoplanets in Andromedagalaxy could be created with it. And they found that the microline event PA-99-N2 from Andromedagalaxy could have actually been such an exoplanet.
A search, however, would take up a lot of time and, above all, relatively large telescopes, only to be able to make a statistical statement afterwards: No one has yet been able to warm up to this astronomical hard work.
We should head to the Magellanic clouds.
Statistics about excalactic exoplanets sound scientifically solid, but little sexy. A single exoplanet beyond our galaxy would be nice. If you then find it with the help of gravitational waves, it would be much nicer, although some astronomers may grumble that the gravitational waves, which are first proven in 2015, are often seen as a kind of panacea for astronomical ailment of all kinds.
But maybe he could work, the suggestion of Camilla Danielski and Nicola Tamanini. In a specialist article published in 2019, the astronomer from the Instituto de Astrofísica de Andalucía and the physicist from the Laboratoire des Deux Infinis in Toulouse suggest the following scenario: Take a double system from two surrounding white dwarfs. This double system generates gravitational waves. If it circles by an exoplanet, this should create slight periodic disorders in the gravitational wave signal.
Danielski and Tamanini calculated that exotic exoplanets could be detected not only within our own galaxy, but also in the Magellanic Clouds. The Magellanic Clouds are satellite galaxies of our Milky Way, not quite as magnificent as the spiral-armed Andromeda Galaxy, but at least definitely extragalactic.
The downside: Currently this is not working yet. Because white dwarfs are so low in mass compared to their related stellar bodies, the black holes and neutron stars, that the gravitational wave signal would be very weak. And then you would have to filter out a small periodic fluctuation in it, which indicates an exoplanet.
The European space organization ESA does not expect Lisa to start the necessary gravitational wave observatory before 2034.
Billions of light-years away, billions of extragalactic exoplanets?
If this takes too long, you could also fall on a specialist article by researchers from the University of Oklahoma from 2018 and come to the end: Why are there still exotractic exoplanets and Nöcher, at least in any galaxy in around four billion light years away? Xinyu Dai and his colleague Eduardo Guerras come to the conclusion that there are exoplanets - around 2000 freely flying planetary objects per star. That would be, you can write it that way, a lot.
Dai and Guerras claim to have detected this planetary population with the help of microlensing with quasars. This technique is also based on gravitational lensing. The background source is a quasar, the "lens" in this scenario is the galaxy about four billion light-years away. The lens reveals itself not only as a variation in brightness in the light of the quasar, but by possibly shifting some of its emission lines: those lines that indicate the presence of chemical elements in the quasar's vicinity, in this case an emission line of neutral iron.
According to the two researchers, part of this shift cannot be explained exclusively by the stars of the galaxy. But it would also need a lot of free-flying, unbound exoplanets. But is it really possible to deduce extragalactic exoplanets by this exclusion procedure?
"I think this claim is quite exaggerated," says Joachim Wambsganß from Heidelberg University. »With the microgravitation lens effect of quasars, it is not possible to determine the mass of the lenses directly. A whole range of other explanatory options can also be responsible for shifting the spectral lines. I think you should provide exceptionally strong evidence for such extraordinary claims. Otherwise that can harm science rather than benefit. "
Off to the extragalactic candidate M51-ULS-1 b in the Whirlpool Galaxy
There are a lot of other galaxies beyond the Andromeda Galaxy and the Magellanic clouds. For example, the whirlpool galaxy M51, which lives up to its nickname and looks truly magnificent, out there about 30 million light-years away in the constellation of Hunting Dogs.
Rosanne di Stefano, astrophysicist at the Harvard-Smithsonian Center for Astrophysics, and her team have recently made a possible candidate in the whirlpool galaxy with the help of data from the Chandra X-ray telescope: M51-ÜLS-1 b. "He would probably be smaller than Saturn, but bigger than Neptune," she says. This exoplanet would also be a real exotic, because it would circle an X-ray sealing star called M51-ULS-1. This X -ray stop star consists of an extremely dense object, probably a neutron star or a black hole. His star partner "donates" to this dense object matter. The processes that take place are so energy -rich that the entire system lights up brightly in the X -ray light.
The planetary contender is said to have betrayed itself by its transit, when the brightness of M51-ULS-1 briefly decreased and then increased again. From this, the researchers calculated a probability distribution of its size as well as its speed. Since this appears to be relatively slow with respect to the source of the X–ray light at 17 kilometers per second – after all, the Earth orbits the Sun at around 30 kilometers per second - Di Stefano's team concluded that the hypothetical planet should orbit the X-ray binary star once every 68 years or so.
But that also means: to check whether this excalactic planet really exists, you would have to wait at least 68 years for it to pass on the X -ray source again. And then another 68 years to really make sure. And the principle of science applies just as long: once is not.
It may take decades to find extragalactic planets.
The first extragalactic exoplanet has thus far been undiscovered.
But what is this search for all these excalactic exotic? Exoplanet by white dwarfs, exoplanet about X -rays, exoplanet, who simply fly through space here and all alone - couldn't you just look for a normal star for a normal exoplanet? After all, there was also the Nobel Prize in Physics for exactly one of the same discovery in the Milky Way in 2019. Even before Nobel Prize winners Didier Queloz and Michel Mayor were able to prove in 1995 that there were planets in the orbits for different suns, other research groups had discovered exoplanets. Only those exoplanet pulsare circled, i.e. dead worlds. Of course, this is not as interesting as a star system that you could say latent similarity to our own sun.
"Statistically, we were able to show ten years ago that every star in the Milky Way has a planet. But we only know about 5,000 of them individually, because discovering exoplanets is still extremely difficult," says Joachim Wambsganß. This is partly due to the fact that exoplanets usually only reveal themselves indirectly via very small changes in the light of their central star:
For this reason, Wambsganß is skeptical about previous proclamation of potential exoplanets in other galaxies, and estimates that it could take many decades for the technology to be so far that you can really prove it safely. In the meantime, Rosanne Di Stefano and her team want to search for other indications. She doesn't have enough time to wait for a renewed transit of a perhaps existing exoplanet M51-ULS-1 B. But the galaxies out there are full of X -ray sources. If there are really exoplanets there, then similar signals would have to be found at least some of them.
"When looking for extragalactic planets, one should ask oneself an honest question: why don't you take a look around our galaxy. Would you doubt that this is also the case in another galaxy?", says Philippe Jetzer. He cites the potential lack of knowledge gain as one reason why the hunt for extragalactic planets is more of a niche in astronomical research. For example, researchers may disagree about whether this or that candidate should pass as a candidate at all: perhaps the possible transit in the Whirlpool galaxy is a dwarf star. Or a brown dwarf. Or just a signal that happens to look like a transit.
However, what all agree: that there must be the extra -alactic exoplanets - somewhere out there, beyond the Milky Way.
